The thermal expansion of fullerite C60 has been measured in the temperature range 2–9 K. A compacted fullerite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below ~ 3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above 5 K our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed

The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.

The linear thermal expansion of compacted Ar-doped fullerite C60(ArxC60) is investigated at 2–12 K using a dilatometric method. The thermal expansion of ArxC60 is also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in a considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed.

The linear thermal expansion of compacted fullerite C60 alloyed with argon (ArxC60) and neon (NexC60) are investigated by a dilatometric method. The experimental temperature is 2–12 K. In the same temperature interval the thermal expansion of ArxC60 and NexC60 are examined after partial desaturation of the gases from fullerite. It is found that Ar and Ne alloying affects the temperature dependence of the thermal expansion coefficient of C60 quite appreciably. The libration and translation contributions to the thermal expansion of pure C60 are separated. The experimental results on the thermal expansion are used to obtain the Debye temperature of pure C60. The effects observed are tentatively interpreted.

The heat capacity C of fullerite doped with deuteromethane (CD4)(0.4)(C-60) has been investigated in the temperature interval 1.2-120K. The contribution Delta C-CD4 of the CD4 molecules to the heat capacity C has been isolated. It is shown that at T approximate to 120K the rotational motion of CD4 molecules in the octahedral voids of the C-60 lattice is weakly hindered. When the temperature is lowered to 80K, the rotational motion of the CD4 molecules changes from weakly hindered rotation to libration. In the range T = 1.2-30 K, Delta C-CD4 is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD4 molecules. The contribution of tunneling rotation to the heat capacity Delta C-CD4(T) is dominant below 5K. The effect of nuclear-spin conversion of the CD4 molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T < 5K have been estimated. A feature observed in Delta C-CD4(T) near T = 5.5K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677237]

The specific heat at constant pressure C(T) of bundles of single-walled carbon nanotubes (SWNTs) closed at their ends has been investigated in the temperature interval of 2–120 K. It is found that the curve C(T) has features near 5, 36, 80, and 100 K. The experimental results on the C(T) and the radial thermal expansion coefficient α_R(T) of bundles of SWNTs oriented perpendicular to the sample axis have been compared. It is found that the curves C(T) and α_R(T) exhibit a similar temperature behavior at T > 10 K. The temperature dependence of the Grüneisen coefficient γ(T) has been calculated. The curve γ(T) also has a feature near 36 K. Above 36 K the Grüneisen coefficient is practically independent of temperature (γ ≈ 4). Below 36 K, γ(T) decreases monotonically with lowering temperature and becomes negative at T < 6 K.

The heat capacity of the interstitial solid solution (CH₄)_0.4C₆₀ has been investigated in the temperature interval 1.4–120 K. The contribution of CH₄ molecules to the heat capacity of the solution has been separated. The contributions of CH₄ and CD₄ molecules to the heat capacity of the solutions (CH₄)_0.40C₆₀ and (CD₄)_0.40C₆₀ have been compared. It is found that above 90 K the character of the rotational motion of CH₄ and CD₄ molecules changes from libration to hindered rotation. In the interval 14–35 K the heat capacities of CH₄ and CD₄ molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to mainly the difference in the frequencies of local translational and libration vibrations of molecules CH₄ and CD₄. The contribution of the tunnel rotation of the CH₄ and CD₄ molecules to the heat capacity is dominant below 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH₄ and CD₄ molecules. The conversion rate of CH₄ molecules is several times lower than that of CD₄ ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH₄ and CD₄ molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.

The heat capacity at constant pressure of fullerite C60 has been investigated using an adiabatic calorimeter in a temperature range from 1.2 to 120 K. Our results and literature data have been analyzed in a temperature interval from 0.2 to 300 K. The contributions of the intramolecular and lattice vibrations into the heat capacity of C60 have been separated. The contribution of the intramolecular vibration becomes significant above 50 K. Below 2.3K the experimental temperature dependence of the heat capacity of C60 is described by the linear and cubic terms. The limiting Debye temperature at T → 0 K has been estimated (Θ0=84.4 K). In the interval from 1.2 to 30K the experimental curve of the heat capacity of C60 describes the contributions of rotational tunnel levels, translational vibrations (in the Debye model with Θ0=84.4 K), and librations (in the Einstein model with ΘE,lib=32.5 K). It is shown that the experimental temperature dependences of heat capacity and thermal expansion are proportional in the region from 5 to 60K. The contribution of the cooperative processes of orientational disordering becomes appreciable above 180 K. In the high-temperature phase the lattice heat capacity at constant volume is close to 4.5 R, which corresponds to the high-temperature limit of translational vibrations (3 R) and the near-free rotational motion of C60 molecules (1.5 R).

The conductivity of bundles of carbon single-walled nanotubes with metallic conductivity (metallic nanotubes) is investigated over the wide temperature range 4.2-330 K and electrical fields up to 50 V. The usage of short electrical pulses of the duration of 10 ns allowed to avoid an influence of a self-heating of the investigated structures on current-voltage characteristics. It is shown that the temperature dependence of conductivity is described by the power function G proportional to T(alpha). At helium temperatures the asymptotic dependence of current on applied voltage is close to J proportional to V(1+alpha) with alpha = 0.45. From comparison of the obtained results of measurements with calculations, it is shown that the conductivity of nanotube bundles is well described within the theory of the Luttinger-liquid conductivity for one-dimensional conductors. The self-heating of the carbon nanotube bundles was observed in the case of measurements in the regime of dc current. A method for determination of the self-heating temperature of nanotube bundles as a function of an applied electrical field is proposed. The power dependence of the self-heating temperature on voltage T proportional to V(p) with the exponent p = 2.1 was observed above some threshold voltage in the temperature range 4.2-200 K. Above 200 K the exponent decreased down to p = 1.35. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3662331]

The effect of oxygen impurities upon the radial thermal expansion αr of bundles of closed single-walled carbon nanotubes has been investigated in the temperature interval 2.2–48 K by the dilatometric method. Saturation of bundles of nanotubes with oxygen caused an increase in the positive α_r-values in the whole interval of temperatures used. Also, several peaks appeared in the temperature dependence α_r(T) above 20 K. The low temperature desorption of oxygen from powders consisting of bundles of single-walled nanotubes with open and closed ends has been investigated.

The radial thermal expansion αr of bundles of single-walled carbon nanotubes saturated with 3He up to the molar concentration 9.4% has been investigated in the temperature interval 2.1–9.5 K by high-sensitivity capacitance dilatometry. In the interval 2.1–7 K a negative αr was observed, with a magnitude which exceeded the largest negative αr values of pure and 4He-saturated nanotubes by three and two orders of magnitude, respectively. The contributions of the two He isotope impurities to the negative thermal expansion of the nanotube bundles are most likely connected with the spatial redistribution of 4He and 3He atoms by tunneling at the surface and inside nanotube bundles. The isotope effect turned out to be huge, probably owing to the higher tunneling probability of 3He atoms.

The radial thermal expansion coefficient alphar of pure and Xe-saturated bundles of single-walled carbon nanotubes (CNTs) is measured in the interval 2.2–120 K. The coefficient is positive above T=5.5 K and negative at lower temperatures. The experiment was done using a low-temperature capacitance dilatometer with a sensitivity of 2×10−9 cm, and the sample was prepared by compacting a CNT powder in such a way that the pressure applied oriented the nanotube axes perpendicular to the axis of the cylindrical sample. The data show that individual nanotubes have a negative thermal expansion, while the solid compacted material has a positive expansion coefficient due to expansion of the intertube volume in the bundles. Doping the nanotubes with Xe caused a sharp increase in the magnitude of alphar in the whole range of temperatures used and gave rise to a peak in the dependence alphar(T) in the interval 50–65 K. A subsequent decrease in the Xe concentration lowered the peak considerably but had little effect on the thermal expansion coefficient of the sample outside the region of the peak. The features revealed are explained qualitatively.

The effect of a N2 impurity on the radial thermal expansion coefficient αr of single-walled carbon nanotube bundles has been investigated in the temperature interval 2.2–43 K by the dilatometric method. Saturation of nanotube bundles with N2 caused a sharp increase in the positive magnitudes of αr in the whole temperature range used and a very high and wide maximum in the thermal expansion coefficient αr(T) at T ~ 28 K. The low temperature desorption of the impurity from the N2-saturated powder of bundles of single-walled carbon nanotubes with open and closed ends has been investigated.

The effect of a normal H2 impurity upon the radial thermal expansion ar of single-walled carbon nanotube (SWNT) bundles has been investigated in the interval T = 2.2–27 K using the dilatometric method. It is found that H2 saturation of SWNT bundles causes a shift of the temperature interval of the negative thermal expansion towards lower (as compared to pure carbon nanotubes) temperatures and a sharp increase in the magnitude of ar in the whole range of temperatures investigated. The low temperature desorption of H2 from a powder consisting of bundles of SWNTs, open and closed at the ends, has been investigated.

The radial thermal expansion α_r of bundles of single-walled carbon nanotubes saturated with ⁴He impurities to the molar concentration 9.4% has been investigated in the interval 2.5–9.5 K using the dilatometric method. In the interval 2.1–3.7 K α_r is negative and is several times higher than the negative α_r for pure nanotube bundles. This most likely points to ⁴He atom tunneling between different positions in the nanotube bundle system. The excess expansion was reduced with decreasing ⁴He concentration.

The thermal expansion of CD4 solutions in the orientational glass C60 with molar concentration of deuteromethane 20 and 50% has been investigated in the temperature range 2.5–23 K. The orientational glass CD4–C60 undergoes a first-order phase transition in the temperature interval 4.5–55 K. This transition is manifested as hysteresis of the linear thermal expansion coefficient alpha as well as maxima in the temperature dependences alpha(T) and tau1(T), where tau1 is the characteristic thermalization time of the experimental samples. The characteristic re-orientation times of the C60 molecules and the characteristic phase transformations occurring in the experimental solutions are determined. The results of the present study are compared with the results of a similar study of the solution CH4–C60. It is concluded that tunneling rotation of the CH4 and CD4 molecules occupying interstitial positions in the fullerite C60 lattice occurs.

The temperature dependence of the coefficient of linear thermal expansion alpha of O-2-C-60 solutions with 20% and 80% filling of the octahedral cavities with oxygen is investigated in the temperature interval 2.2-24 K. Hysteresis of alpha(T) is observed, attesting to the coexistence of two orientational glasses in these solutions. A comparison of the behavior of these glasses is made. The characteristic times for reorientation of the C-60 molecules and for the phase transformations in the solutions are determined. When the temperature of the O-2-C-60 solution with 20 mol.% oxygen is increased to 450 degrees C, a chemical interaction of the oxygen with the C-60 molecules is manifested. It proves possible to separate the influences of the chemical and physical sorption of oxygen on the thermal expansion of polycrystalline fullerite C-60.

A brief review is given of the interaction between fullerite C₆₀ and various gases under elevated pressure. Subjects discussed include the formation of ordered interstitial gas-fullerene compounds, reactions between intercalated gases and fullerene molecules to form new endohedral and exohedral compounds, and changes in the structure and properties of C₆₀ because of intercalated gas atoms or molecules.